1. Field of the Art
This invention relates generally to fabrication by press-molding of precision optical glass elements such as optical glass lenses or the like, and more particularly to a transfer mechanism for transferring work pieces of optical glass material each having curved surfaces on the opposite sides, accurately from a work container like a pallet to a predetermined position on a mold assembly unit to be used in press-molding.
2. Prior Art
For fabrication of precision optical glass elements like lenses, for example, press-molding processes are increasingly resorted to more than ever. Generally, an optical glass press-molding apparatus has a construction as shown in FIG. 12. In that figure, indicated at 1 is a mold assembly unit, at 2a is an upper pressing member, and at 2b a lower pressing member. In this case, upper and lower pressing members 2a and 2b constitute a press means. The mold assembly unit 1 is largely composed of an upper mold member 3, a lower mold member 4 and a girdler shell 5. Normally, the lower mold member 4 is fixedly assembled into the girdler shell 5, while the upper mold member 3 is movable toward and away from the lower mold member 4 under the guidance of the girdler shell 5.
In a preparatory stage prior to press-molding, the upper mold member is 3 is once separated from the lower mold member 4, and, after setting optical glass material 6 on a glass shaping surface 4a of the lower mold 4, the upper mold member 3 is closed again on the lower mold 4. Nextly, the mold assembly 1 as a whole is heated by the use of heating means 7 to soften the glass material 6, and at the same time the upper and lower molds 3 and 4 are pressed toward each other by the upper and lower pressing members 2a and 2b. As a consequence, the optical glass material 6 in the mold 1 is pressed to shape to produce an optical glass element 6 which have predetermined surface characteristics copied from glass molding surfaces 3a and 4a of the upper and lower mold members 3 and 4.
In order to carry out the lens molding operation automatically, for instance, a lens molding apparatus basically of the above-described construction can be incorporated into an automatic lens molding line in the manner as shown schematically in FIG. 13. In that figure, indicated at 10 is a molding chamber which is equipped with a heating system 7 along with upper and lower pressing members 2a and 2b. Indicated at 11 is an entrance/exit way or station through which a mold assembly unit 1 with optical glass material 6 is loaded into the molding chamber 10 or to which a mold assembly unit 1 with a molded glass product is delivered from the molding chamber 10 after press-molding therein. Indicated at 12 is an upper mold assembling/dissembling station, at 13 a work loading/unloading station. At the work loading/unloading station, optical glass material 6 is placed on a lower mold member 4, or an optical lens element which has been fabricated by press-molding is ejected from a lower mold. Accordingly, at the upper mold assembling/dissembling station 12, an upper mold member 3 is removed from a mold assembly unit 1 on the way to the loading/unloading station 13. Conversely, an upper mold member is set on a mold assembly unit 1 which arrives from the loading/unloading station 13. The mold assembly unit 1 is supported on a suitable transfer jig and thereby transferred horizontally to and from the above-mentioned stations.
First of all, a work loading pallet in the form of a container which is arranged to hold a large number of pieces of the optical glass material 6 is located at the loading/unloading station 13 along with a work unloading pallet or a jig which is arranged to hold pieces of molded lens product thereon. Besides, for automatic loading and unloading operations, a robot with a suction arm is provided at the work loading/unloading station to transfer pieces of optical glass material 6 onto the loading pallet and to pick up a molded lens product 8 from a mold assembly unit 1. This is the main reason why an upper mold member is put on or off at the upper mold assembling/dissembling station. An upper mold member 3 is picked up and retained on a holder member at the upper mold assembling/dissembling station 12, and a lower mold assembly consisting of a lower mold member 4 and a girdler shell 5 is sent to the loading/unloading station 13 with a molding surface 4a faced upward in an open state. As soon as optical glass material 6 is placed in the lower mold 4, the lower mold assembly is transferred to the upper mold assembling/dissembling station to receive an upper mold member 3 into the girdler shell 5. Then, the mold assembly unit 1 is sent into the molding chamber 10 through the entrance/exit way 11, and, while being softened under heating by the heating mechanism 7, the optical glass material 6 is pressed to shape by the press means.
Upon finishing the press-molding of a lens element within the molding chamber 10, the mold assembly unit 1 is transferred to the entrance/exit way 11 and then to the upper mold assembling/dissembling station 12 to remove the upper mold member 3 off the lower mold 4 and out of the girdler shell 5. The lower mold assembly, with a molded lens element 8 on the opened molding surface 4a of the lower mold 4, is further transferred to the work loading/unloading station where the molded optical lens product on the lower mold 4 is ejected therefrom and replaced by fresh optical glass material 6. These operations are repeated to mold optical lens elements automatically and continuously.
In this connection, the shape of optical glass material is determined in relation with the shape of optical lens elements of the end product. For example, in the case of an optical lens product with a large radius of curvature, the optical glass material can be spherical or nearly spherical in shape. However, in the case of an optical lens product with a small radius of curvature, individual work pieces of optical glass material should be of a somewhat flattened shape having curved surfaces of a predetermined radius of curvature instead of surfaces of spherical shape. At the time of press-forming optical glass material having such flattened curved surfaces, a center O1 of curvature on a front side 6a of glass material 6 as well as a center of curvature O2 on a rear side 6b of the glass material should be positioned exactly on an axis A which connects the centers of curvature O3 and O4 of molding surfaces 3a and 4a of the upper and lower molds 3 and 4 as shown in FIG. 14. If the centers of curvature O1 and O2 on the front and rear sides of the optical glass material is deviated from the axis A, pressure is non-uniformly applied to the glass material in the pressing stage to produce a lens element which is distorted in optical characteristics.
In the work loading stage, the optical glass material 6 is picked up from a pallet by a robot with a suction gripper means or the like and set on the shaping surface 4a of the lower mold 4. At the time when the optical glass material 6 is released from the suction gripper, compressed air is usually blasted on the glass material in order to transfer same onto the lower mold 4 positively or in a forced way. Therefore, positional deviations may occur to the optical glass material 6 when it is transferred in this manner, more particularly, when it is picked up and also when it is set on the lower mold 4. Especially, since the lower mold 4 is circumvented by the girdler shell 5, the pressure of compressed air which is used to blow the optical glass material off the suction gripper means can find no way to escape and acts on the optical glass material directly and repeatedly to cause positional deviations to the latter, shifting the centers of curvatures O1 and O2 on the front and rear surfaces of the optical glass material 6 away from the axis A.